Thoracic Sympathetic Nervous System: Statement Analysis
Statement-by-Statement Evaluation
1. Cardiac outflow is from the upper four or five thoracic spinal cord segments
FALSE
The cardiac sympathetic outflow originates from a broader range of thoracic segments than stated. 1 Sympathetic neurons originate from the reticular formation in the brainstem and project to postganglionic neurons, with the thoracolumbar sympathetic outflow traditionally described as spanning T1-L2. 2 The preganglionic neurons supplying cardiac function are distributed across multiple thoracic levels, typically T1-T5 or T6, not limited to just the upper four or five segments. 3
2. Postganglionic fibres from the cardiac plexuses cause constriction of the coronary arteries
FALSE
This statement oversimplifies and misrepresents the sympathetic effects on coronary vessels. 1 While sympathetic efferent neurons do provide control over coronary vessels, the primary cardiac sympathetic effects include increased heart rate, enhanced contractility, and improved conduction velocity. 4 The coronary arteries possess both alpha-adrenergic receptors (causing vasoconstriction) and beta-2 adrenergic receptors (causing vasodilation), with the net effect during sympathetic activation typically being vasodilation to meet increased myocardial oxygen demand, not pure constriction.
3. Most of the ganglia are in front of the heads of ribs
TRUE
The sympathetic chain ganglia in the thoracic region are anatomically positioned anterior to the heads of the ribs, lying along the lateral aspect of the vertebral bodies. 2 This anatomical arrangement is consistent throughout the thoracic sympathetic trunk, where the ganglia are located in the paravertebral position, anterior to the costovertebral articulations.
4. Splanchnic nerves carry preganglionic fibres
TRUE
The splanchnic nerves (greater, lesser, and least) are composed of preganglionic sympathetic fibers that originate from the intermediolateral cell column of the thoracolumbar spinal cord. 2, 3 These preganglionic fibers pass through the sympathetic chain without synapsing and travel to prevertebral ganglia (celiac, superior mesenteric, and aorticorenal ganglia) where they synapse with postganglionic neurons. 5 This is a fundamental organizational principle of the sympathetic nervous system's visceral innervation.
5. Greater splanchnic nerves pass behind medial arcuate ligaments
TRUE
The greater splanchnic nerves pierce the diaphragm by passing posterior to (behind) the medial arcuate ligaments to reach the celiac ganglia in the abdomen. 2 This anatomical relationship is consistent, with the nerves traversing the diaphragm in the posterior mediastinum before synapsing in the prevertebral ganglia.